It is known from application WO2010/049871 a field effect transistor comprising two dielectric layers, two control or gate electrodes, and an assembly consisting of a source electrode or source, a drain electrode or drain, and an organic semiconductor in contact with said source and drain. Such an assembly is positioned between said two dielectric layers, each of which is positioned between said assembly and a control electrode. A light emitting transistor comprising such transistor is disclosed, wherein said organic semiconductor is an ambipolar organic semiconductor layer. The thickness of said semiconductor layer is necessarily limited to a few molecular layers, and preferably said thickness of the semiconductor layer is less than 10 nm, to allow the radiative recombination of electrons and holes, that were injected respectively from the source and drain and transported to the two interfaces of the semiconductor layer with the dielectric layers between which semiconductor said layer is arranged.
However, the emission properties of the light emitting transistor according to application WO2010/049871 are limited by intrinsic factors.
As a matter of fact, due to the above mentioned dimensional constraint on the maximum thickness of the semiconductor layer of the known transistor, the volume of semiconductor material in which the radiative recombination of the charges takes place is small and the intensity of light emission is consequently limited.
In addition, in the known transistor a single semiconductor layer is responsible for transport of both electrons and holes, thus limiting in practical applications the electrical performance of the device.
Moreover, the light emitting transistor according to application WO2010/049871 has a limited flexibility concerning the driving of the device.
It is also known from US2009/0008628 a field effect transistor comprising two dielectric layers, two control or gate electrodes, an assembly positioned between the two dielectric layers consisting of two transporting layers, an emissive layer positioned between the two transporting layers and source and drain electrodes, which are either both in contact with both conducting layers by interaction between the vertical surface of the contacts and the vertical surface of the conductive layers, or which have the vertical surface of one electrode (source or drain) in contact with the vertical surface of one conducting layer and the vertical surface of the other electrode (drain or source) in contact with the vertical surface of the other conducting layer.
The practical applications of the organic field effect transistors according to US2009/0008628 are affected by the transport layers/source and drain contacts geometry, which limits the efficiency of charge injection into the transport layers and therefore the overall electrical characteristics of the device. In addition, as a matter of fact, the practical realization of field effect transistors according to US2009/0008628 is unlikely to be realized with satisfactory quality and industrial reproducibility yields using standard manufacturing techniques. Bad contact points and shadow effects between the vertical surface of the conducting layers and the contacts are likely to be generated during the fabrication process, because the multilayer structure of US2009/0008628 requires a hole transporting layer, a light emitting layer, an electron transporting layer and a second insulating layer to be sequentially formed between the source electrode and the drain electrode in a direction parallel to these electrodes.
It is an objective of the present invention to provide an electroluminescent organic transistor which is free from said drawbacks. Said objective is achieved with an electroluminescent organic transistor whose main features are specified in the first claim and other features are specified in the remaining claims.
A first advantage of the electroluminescent organic transistor according to the present invention consists in its improved emission characteristics compared to the single layer transistors of the prior art. As a matter of fact, a material specifically dedicated to the emission of light, having an excellent efficiency in the generation of light, is provided in the ambipolar channel of the transistor according to the present invention.
In addition, the emissive layer in the channel of the electroluminescent organic transistor according to the present invention may have a larger thickness than the semiconductor layer in which the recombination occurs in the known single layer transistor, therefore the intensity of light emission in the device according to the present invention is higher compared to that of the known single layer transistor.
A further advantage of the electroluminescent organic transistor according to the present invention compared to that of the known single layer transistor consists in the fact that an optimization of the charge transport is allowed. As a matter of fact, thanks to the presence of two control electrodes and of an ambipolar channel comprising two semiconductor layers, each optimized for the transport of only one type of charge, the differences in the mobility of the charges and in the current density in said two semiconductor layers may be more effectively balanced by means of a suitable modulation of the potentials of the two control electrodes.
A further advantage of the electroluminescent organic transistor according to the present invention compared to that of the known trilayer transistor consists in the more effective charge injection into the active channel of the device, which leads to overall higher electronic and optoelectronic performances.
The electroluminescent transistor according to the present invention can be driven either in direct or reverse mode. In fact, in the device according to the present invention the charges can be transported not only at the interface between the semiconductor layers and dielectric layer, but also at the interface between the semiconductor layers and emissive layer, with a direct benefit on the emission efficiency and intensity